1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing unit which is configured to fix a toner image to a sheet by fusing the unfixed toner while the sheet is passed through a nip between a pair of heated rollers or between a heating belt and a roller.
2. Description of the Related Art
In the aforementioned kind of image forming apparatus, fixing belt systems attract attention due to growing demand for a reduction in warm-up time of a fixing unit and energy savings in recent years. This is because a fixing belt has a low heat capacity as mentioned in Japanese Unexamined Patent Publication No. 6-318001, for example. Also attracting attention recently is electromagnetic induction heating (IH) technology which offers a high-speed, high-efficiency heating capability. Today, products developed by using a combination of the IH technology and belt systems for achieving energy savings in a process of fusing color toner images are available in large quantities on the market. An arrangement widely used combining the IH technology and belt systems is to dispose an induction heating element on the outside of the heating belt (known as an external IH system). The external IH system is often used because this arrangement provides such advantages as ease of layout and cooling of an induction coil and a capability to directly heat the heating belt.
In practical applications of the IH technology, there exist various arrangements devised for preventing overheating of non-sheet passing areas of a fixing roller of a fixing unit according to the width (sheet passing width) of each sheet of paper passed through the fixing unit. For example, Japanese Unexamined Patent Publication No. 2003-107941 and Japanese Patent No. 3527442 introduce means for altering a heated area of a fixing roller according to the sheet passing width. These means of the prior art (hereinafter referred to as first and second prior art arrangements) intended particularly for external induction heating are configured as briefly described hereunder.
The first prior art arrangement shown in Japanese Unexamined Patent Publication No. 2003-107941 applied to a fixing unit includes a magnetic member, an exciting coil and a moving mechanism. The magnetic member is divided into a plurality of pieces which are arranged along a sheet passing width direction, and the moving mechanism moves part of the magnetic member toward and away from the exciting coil according to the width of each sheet passed through the fixing unit. It is supposed that an effect of this arrangement is to decrease heating efficiency in a non-sheet passing area by separating the magnetic member from the exciting coil, thus reducing the amount of heat generated in the non-sheet passing area than in an area corresponding to a minimum sheet passing width.
The second prior art arrangement shown in Japanese Patent No. 3527442 applied to a fixing unit is such that an additional electrically conductive member is disposed within a heating roller in an area outside a minimum sheet passing width, wherein this electrically conductive member is made movable between a position within a range of a magnetic field and a position outside the range of the magnetic field. In this prior art arrangement, the heating roller is preheated by induction heating with the electrically conductive member initially arranged outside the range of the magnetic field. When the heating roller is heated almost up to the Curie temperature, the electrically conductive member is moved to the outside of the range of the magnetic field, causing magnetic flux to leak from the heating roller outside the minimum sheet passing width to prevent overheating.
In the first prior art arrangement, the magnetic member should have a large movable range, so that this arrangement has a problem that the entirety of the fixing unit becomes unnecessarily large. On the other hand, the second prior art arrangement offers a space-saving capability because means for altering a heated area is provided in an internal space of the heating roller. The internal space of the heating roller is however a high-temperature environment. Therefore, if some kind of component is mounted inside the heating roller, it is necessary to increase the Curie temperature of the heating roller and, in addition, there arises a problem that the provision of a large-sized component having a large heat capacity within the heating roller causes an increase in warm-up time thereof.